A user equipment with device mobility often moves from one geographical location to another. Whenever a user equipment (UE) served under the domain of one base station migrates to the domain of another base station, the serving core network must have a way to keep track of the UE in case that the UE needs to be contacted by a phone call or a message. Since the UE could move between cells, paging all the cells in existence would be unnecessary and thus a way of keeping track of the UE by the serving core network.
Tracking areas are used to keep track of approximate locations of UEs served by a network. Each tracking area would have a unique global identity which is the tracking area identity (TAI). A tracking area would contain one or more cells, and a network would normally allocate to each UE a list known as a TAI list having one or more tracking areas. The TAI list is normally synchronized between a UE and a Mobility Management Entity (MME). When the network has to reach a UE such as when a call is incoming, the network would attempt to page the tracking areas in the TAIs list which was provided for the UE so that the UE could be located without the network having to page each and every possible cells in existence. Whenever a UE migrates to a tracking area which is not a tracking area of the TAI list, a TA update (TAU) would need to occur between the MME and the UE. Therefore, the TAI list is essentially a list of TA's where a mobile UE may move about without having to updating the MME.
FIG. 1 illustrates a first tracking area 101, a second tracking area 102, and a third tracking area 103. For LTE, these track areas 101, 102, 103 could be connected to a MME through a backhaul link such as a S1 interface. When the UE 104a first attach to one of the base stations or eNBs from the first tracking area 101, the MME would send a TAI list which includes tracking area 101. In step S121 when the UE 104a migrates to the second tracking area as UE 104b, a first TAU, or TAU1, would be performed as the MME would send a new TAI list including tracking areas 101 and 102. In step S122 when the UE 104b migrates to the third tracking areas as UE 104c, a second TAU, or TAU2, would be performed as the MME would send a new TAI list including track areas 101, 102, and 103 to the UE 104c. The TA updates as illustrates in FIG. 1 is summarized by the table 131 of FIG. 1.
In a live network, a UE's response for an ATTACH ACCEPT/GUTI REALLOCATION COMMAND/TRACKING AREA UPDATE ACCEPT messages is according to 3Gpp TS 24.301 specification. The version of the 24.301 specification at this time has mentioned that the MME would assign and include the TAI list the UE is registered to in the ATTACH ACCEPT/GUTI REALLOCATION COMMAND/TRACKING AREA UPDATE ACCEPT messages. FIG. 2A illustrates an ATTACH ACCEPT message content which contains a TAI list information element (IE) 201. FIG. 2B illustrates the content of the TAI list IE. The UE upon receiving an ATTACH ACCEPT message would delete its old TAI list and store the received TAI list.
Also, currently the UE is required to keep track of a list of forbidden tracking areas or a forbidden TAI list. Under section 5.3.2, it mentions “ . . . The UE shall store a list of “forbidden tracking areas for roaming”, as well as a list of “forbidden tracking areas for regional provision of service”. These lists shall be erased when the UE is switched off or when the UICC containing the USIM is removed, and periodically (with a period in the range 12 to 24 hours). When the lists are erased, the UE performs cell selection according to 3GPP TS 36.304 [21]. One or more tracking areas is removed from the list of “forbidden tracking areas for roaming” in the UE, as well as the list of “forbidden tracking areas for regional provision of service” if, after a subsequent procedure e.g. attach procedure, tracking area updating procedure and GUTI reallocation procedure, one or more tracking areas in the lists is received from the network. If the UE has only one PDN connection established which is for emergency bearer services, the tracking areas shall not be removed from these lists if one or more tracking areas in the lists are received from the network. In S1 mode, the UE shall update the suitable list whenever an ATTACH REJECT, TRACKING AREA UPDATE REJECT, SERVICE REJECT or DETACH REQUEST message is received with the EMM cause #12 “tracking area not allowed”, #13 “roaming not allowed in this tracking area”, or #15 “no suitable cells in tracking area”. Each list shall accommodate 40 or more TAIs. When the list is full and a new entry has to be inserted, the oldest entry shall be deleted.
Usually when a UE attempts to attach to a network, the UE would receive the ATTACH ACCEPT from a MME if the current tracking area is not in the forbidden list of the UE. However, currently it could be possible for a tracking area in the TAI list received from MME to be the same as a tracking area in the forbidden TAI list. In that case, when UE roams to the forbidden tracking area, the network might behave unpredictably. This issue would need to be resolved according to a different TAI list updating mechanism.